EU leaders agreed last week it was highly likely Russia was behind the nerve-agent poisoning.

British Prime Minister Theresa May said the show of solidarity gave a strong message.

"We welcome today's actions by our allies, which clearly demonstrate that we all stand shoulder to shoulder in sending the strongest signal to Russia that it cannot continue to flout international law," a statement from Mrs May's office said.

Foreign Secretary Boris Johnson said the "extraordinary international response by our allies stands in history as the largest collective expulsion of Russian intelligence officers ever".

The US action is also the biggest move against Russia since the Cold War era and the hostilities with the then Soviet Union.

The Russian foreign ministry said the moves demonstrated a continuation of a "confrontational path".

"It goes without saying that this unfriendly act by this group of countries will not go without notice and we will react to it," its statement said.

Why are they doing it?

President of the European Council Donald Tusk said 14 EU states had decided to expel Russian diplomats as a direct result of a meeting, held last week, about the Salisbury poisoning.

"Additional measures, including further expulsions within this common EU framework are not to be excluded in the coming days and weeks," he said.

The US state department said in a statement: "On March 4, Russia used a military-grade nerve agent to attempt to murder a British citizen and his daughter in Salisbury.

"This attack on our Ally the United Kingdom put countless innocent lives at risk and resulted in serious injury to three people, including a police officer."

It called the attack an "outrageous violation of the Chemical Weapons Convention and breach of international law".

The US is expelling 48 envoys at the Russian embassy in Washington and 12 more at the UN in New York. It will also order the closure of the Russian consulate in Seattle.